Planetary Evolution and Natural Processes
The Explanation
Earth's habitable environment did not appear all at once in its present form. The planet has changed continuously throughout its history.
Planet formation, atmospheric evolution, volcanic activity, weathering, ocean chemistry, geological cycling, stellar evolution, impacts, and biological activity have all contributed to the environment we observe today.
Some characteristics that may initially appear to be individually arranged for life can therefore arise from physical processes and feedbacks operating over enormous periods of time.
Supporting Evidence
Modern planetary science increasingly treats habitability as dynamic rather than static.
Earth's atmosphere has changed substantially. Its continents and oceans have changed. The Sun itself has changed. Geological and chemical cycles move materials between Earth's interior, surface, oceans, and atmosphere.
Climate feedbacks can counter some environmental changes, while other processes can destabilize planetary environments.
These discoveries demonstrate that habitability must be understood as the history of an interacting planetary system rather than merely as a list of present-day conditions.
Limitations
Explaining how a particular planetary mechanism operates does not necessarily explain why the larger planetary system possesses the properties that allow that mechanism to function.
Natural processes can also produce uninhabitable outcomes. Venus, Mars, and Earth illustrate how planets with some broadly similar beginnings can follow very different environmental histories.
Scientists therefore continue to investigate which combinations of planetary processes tend to establish, preserve, diminish, or destroy habitability.
Questions That Remain
Which of Earth's stabilizing processes are common among rocky planets?
How dependent are those processes upon Earth's particular size, composition, orbit, water inventory, geological activity, and host star?
Could substantially different planetary systems achieve comparable long-term habitability?
Intelligent Design Response
Natural mechanisms and Intelligent Design are not necessarily competing explanations at the same level.
Discovering a physical mechanism can explain how a feature of Earth works. The design question asks a broader question about the origin and organization of the system in which those mechanisms operate.
A responsible design argument should therefore welcome increasingly detailed scientific explanations rather than depend upon gaps in scientific knowledge.